The present invention is directed to a process for forming styrenic polymer beads that form foams having high strength and thermal properties. Such high strength foams are useful, for example, in the production of cups for drinking.
The formation of styrenic polymer beads by an aqueous suspension process is a well known and commercial practice. Such production is generally effected by the suspension polymerization of a styrenic monomer in an aqueous medium with an oil-soluble polymerization catalyst, using a suspending system comprising of a finely divided, difficultly water-soluble inorganic phosphate, such as tricalcium phosphate, in conjunction with an anionic surface-active agent, such as sodium dodecylbenzene sulfonate, as a modifier. Such suspension polymerization systems are described, for example, in Grim Patent, U.S. Pat. No. 2,673,194, the contents of said patent being incorporated by reference herein. Such styrenic polymer beads are made expandable by impregnation with a suitable blowing agent.
The foamed cup industry is based on expandable polystyrene (EPS) beads. The base polystyrenic polymer beads used for cups are prepared by an aqueous suspension polymerization in order that the polystyrenic polymer may be recovered as beads which can be screened to a relatively precise bead size. Because of the thin walls in cups, relatively small head sizes are required in a strict range, such as through 35 on 70 mesh (U.S. Standard Sieve). The conventional polystyrene used for EPS beads for cups are characterized by a weight average molecular weight (Mw) of about 250,000-350,000 with a polydispersity (M.sub.w /M.sub.n) of about 2-3. Such polystyrenes generally have melt flows (M.I., Condition L) in the 2.0-6.0 g/10 min. range. The intrinsic properties of such polystyrenes puts a limit on the molding conditions for producing cups as well as on the cup properties such as strength. Past efforts to utilize higher molecular weight polystyrenes (350,000-500,000) with M.I. of 1-2, with similar M.sub.w /M.sub.n in order to improve on heat sensitivity and cup strength has generally resulted in poor overall performance in regard to cup processing capabilities as well as cup appearance.